This invention relates to an image forming device such as a copying machine and a printer and more particularly to an image forming device equipped with a fixing device which comprises a temperature detecting means which is in contact with the surface of a heating roller to detect its temperature.
Generally, a fixing device of an image forming device is equipped with a means to detect the surface temperature of a heating roller to keep the surface temperature at a preset value. There are two types of temperature detecting means: contact-type temperature detecting means which is in contact with the surface of a heating roller to detect the temperature of the surface and non-contact-type temperature detecting means which detects the temperature of the surface without touching the roller surface.
Concerning the contact-type temperature detecting means, a temperature detecting means such as a thermistor touches the surface of a heating roller and consequently will be covered with offset toner from the heating roller. The offset toner will come off from the temperature detecting means and smudge recording paper.
To solve such a problem, the contact-type temperature detecting means is usually provided in a non-paper-path area which is outside a paper path section where paper touches the heating roller.
Meanwhile, there are two types of non-contact-type temperature detecting means: type to detect infrared rays as temperature and type to detect radiant heat as temperature. A non-contact-type temperature detecting means unlike the contact-type can be provided in the paper path section of the heating roller.
One example of the above-mentioned prior art is disclosed in the Japanese Application Patent Laid-Open Publication No. 2003-98899.